This invention relates to an optical disk drive.
Generally, an optical disk drive writes and reads data on an optical disk by means of a laser beam. The optical disk drive includes a light source module that emits the laser beam and an optical head carrying an object lens that converges the laser beam on a small light spot on the optical disk.
The tracking operation of the optical disk drive includes (1) a rough tracking operation and (2) a fine tracking operation. The rough tracking operation is accomplished by moving the optical head crossing the tracks of the optical disk. The fine tracking operation is accomplished by minutely moving the light spot on the optical disk. For this purpose, a galvano mirror is provided in a light path between the light source module and the object lens. By rotating the galvano mirror, the angle of incidence of the laser beam entering the object lens is changed, so that the light spot on the optical disk is moved.
FIGS. 1A and 1B are a perspective view and a side view of a conventional galvano mirror system. The galvano mirror 44 is mounted to a mirror holder 42. The mirror holder 42 is supported by a stator 41 via plate springs 43 so that the mirror holder 42 is rotatable about a rotation axis P. In order to actuate the galvano mirror 44, a coil 45 is fixed to the mirror holder 42. A magnet yoke 46 is provided to the stator 41, which generates a magnetic field in which the coil 45 is positioned. When current flows in the coil 45, the galvano mirror 44 is rotated as shown by A in FIG. 1B, due to an electromagnetic induction caused by the current flow in the coil 45 and the magnetic field caused by the magnets of the yoke 46.
In order to detect the rotation of the galvano mirror 44, a reflecting surface 50 is formed on one of side faces of the mirror holder 42. Further, a photo-sensor 49 is provided to a circuit board 47 fixed to the stator 41 so that the photo-sensor 49 is faced with the reflecting surface 50. The photo-sensor 49 includes an LED chip 49A and two photo-transistors 49B. The LED chip 49A and the photo-transistors 49B are disposed on a line that is perpendicular to the rotation axis of the mirror holder 42. The two photo-transistors 49B are located on both sides of the LED chip 49A (on the above-mentioned line).
In a state the reflecting surface 50 is in parallel to the photo-sensor 49, the lights emitted from the LED chip 49A and reflected by the reflecting surface 50 equally reach two photo-transistors 49B. When the mirror holder 42 is rotated as shown by A in FIG. 1B, light entering one of the photo-transistor 49B is increased, while light entering the other photo-transistor 49B is decreased. Thus, the rotation amount of the galvano mirror 44 can be detected by measuring the difference in outputs of two photo-transistors 49B.
However, since the reflecting surface is provided to one of side faces of the mirror holder 42, such a conventional galvano mirror system has a disadvantage such that an arrangement of the photo-sensor occupies relatively large space.
Further, if there is a difference in sensitivities of the photo-transistors 49B, it may cause an offset. That is, there is a possibility that the outputs of two photo-transistors 49B are not equal with each other, even if the reflecting surface 50 is in parallel to the photo-sensor 49. In the conventional galvano mirror system, it is difficult to remove such offset.